I'm not sure you can. I can see Sigma Aldrich selling it by the kilogram for research use. I'm not sure there is an approved human formulation. If it's because of that gut paper lactulose is pretty easily available.
Thanks for having a look. Yeah it's because of the paper. I dug into the source and the research they cited, and it sounded a lot like lactulose was pretty much ineffective at promoting any of the really beneficial bacteria. And the price for chemical grade Rhamnose is way beyond my budget, especially as, as far as I could tell, the dosage is pretty high, to get the benefits.